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Steven Moffat, showrunner of the global BBC hit, also discusses the cult show and its future.

LONDON -- BBC evergreen and global hit Doctor Who is celebrating its 50th anniversary with a special that will air on the U.K. public broadcaster and simulcast around the world, including on BBC America, on Nov. 23.

The 75-minute episode, entitled "The Day of the Doctor," was written by showrunner Steven Moffat and features outgoing lead Matt Smith, known as the 11th Doctor, predecessor David Tennant, John Hurt, current sidekick Jenna-Louise Coleman, former sidekick Billie Piper and others.

The special has attracted much speculation and buzz, which is set to continue when Smith leaves the show lead to Peter Capaldi in this year's Christmas special.

The 50th anniversary special was shot this spring in Cardiff, Wales where the production is based. During a set visit by reporters, Moffat and some of the actors talked about the show and what it is like to work on it.

"I knew what i wanted to accomplish," Moffat said. "It is a tremendous challenge to attach 50 to anything. I wanted it to be the show that ensures the next 50 years. And it celebrates the legend" of Doctor Who.

Asked about the special's title, he said: "The question was: what was the most important thing that happened to the Doctor, the day the Doctor won't forget. It will redefine him."

Does that mean the tone is darker? "It's quite a dark episode," Moffat said. "It's quite a serious episode. We start to move the Doctor into a place, and you'll start to see we've been moving him into that direction for a while."

Discussing having his predecessor on set, Smith told reporters: "That's been one of the great treats – acting with David." Was there competition between the two actors or the two Doctors? "They disapprove of each other, they get along. It sort of ebbs and flows," he said. "But only you guys think it's competition [between the actors]. We have a pretty even split of screen time. I think it would have been disappointing not to have another Doctor on."

Tennant agreed that the collaboration was more exciting than tense. "It's his gig," he said about Smith who only later announced his departure from the show, with Peter Capaldi set to take over. "I'm a guest on his show. He's been very enthusiastic about this thing happening."

Coleman said instead of rivalry, the collaboration of Smith and Tennant was great to follow on set. "They complement each other very well," she said.

Asked how the two Doctors are different, Smith said: "David's is more swashbuckling than mine."

Describing the challenge of having two Doctors in the same episode, Moffat said: "The Doctor is the Doctor. He is one man who has different phases and faces. He is the same man inside -- the same man at different stages." So, the actors in the special do some of the same things, then suddenly diverge, he explained.

The two Doctor Who actors also swapped notes, according to Tennant. "It was less on playing the part, but the lifestyle that is inflicted upon you," he explained. "The attention the Doctor gets is relatively unique," he said.

Asked if the two actors compared their Doctors' respective sonic screwdrivers, a key tool used by Doctor Who, Smith quipped: "Yes, mine's bigger."

Confronted with that comment later, Tennant quipped: "Everyone's bigger than mine. Maybe I just don't have to compensate that much."

What did Tennant do to snap back into the role of the Doctor? "I had to go and get a couple of DVDs," he acknowledged. "It just sort of came back. It's something about its energy. It's not naturalistic like most other things are. It's the tone that's unlike anything else. That's what people find when they guest on it. It's been a lovely thing to revisit."

Asked about his return to the set, he said: "Coming back to Wales and seeing the trailer and everything hanging there like [before] was strange. I thought: Did I just dream the last bit?"

Coleman said she also enjoyed the return of the Doctor Who alumni. "I enjoy kind of having a gang," she told reporters. "We were all swapping tales and Doctor Who stories. They've lived through what we're doing now."

Smith shared some insight into what playing Doctor Who means. "It's unexpected to have people dress up as you, watch people have little figures of you," he said. "There are so many things you can't share with anyone. It sounds like bragging or moaning. It's such a dominant experience in your life, and that's hard to communicate to people."

Asked about weird experiences with fans, Smith said: "I have been followed to the toilet by a man, and his wife -- Sheila or so -- was on the phone, and I was having a wee. I said dude this is not the best time for me. Then I washed my hands and said hello."

All actors raved about working with John Hurt. "He is just sorted," Coleman said. "He is really, really cool. You just want to talk to him."

"He is a charming man and a great actor," Smith said. "He's in his 70s, and he just ploughs on. When the camera is on him, that's the only place you want to look."

The 50th anniversary special was filmed in 3D and will be shown in 3D. Did that change the work on set? "The director told us to react to the Doctor pointing at us," she said. "Many will still see it in 2D, so you have the same ingredients of the Doctor Who series. But the show lends itself to 3D."

Overall, she described the experience of shooting in 3D as "better even than I imagined." But progress was slower than normal, she said. "You are doing a scene for a day and a half. There are longer setups."

Discussing his hopes for audiences' reactions after watching the special, Smith said: "I think it's sort of celebratory. I hope audiences think ah, there's two of them, and they're both ridiculous."

Showrunner Moffat was also quizzed about the recipe for keeping Doctor Who a hit. "You have to make it as good and inventive as possible," he said. "And you can never turn cynical and think about making the show cool or hot. I try to look for what would I get excited about. And we make ourselves as excited as eight year olds."

And how long can Doctor Who and his work on it continue? "I honestly don't know," said Moffat who has been involved in the show for 10 years. "I have no intentions to leave."